What 1,000 hornbills contribute to the tiny Narcondam Island
A research team finds these birds help maintain plant diversity—and that we have just one chance to save the island
Oceanic islands harbour some of the most precious living jewels of planet Earth. Narcondam Island in the Andaman Sea was born about 700,000 years ago when a volcano erupted in the middle of the ocean. The ocean around the island is very deep, so it was never connected to the mainland even during the coldest times on Earth, when sea levels were much lower than they are today.
Over the years, plant seeds dropped by passing birds, or carried by the wind or ocean currents, gave rise to the luxuriant forests that are now found on the island. The island is small (6.8 sq. km) but large enough to harbour some fascinating terrestrial wildlife.
Among the different creatures on the island, the most fascinating is a bird found nowhere else in the world. It is the Narcondam hornbill, one of the 32 hornbill species found in Asia. Its ancestor probably colonised the island long ago and it then evolved into a distinct species there. Narcondam hornbills, like the other Asian hornbills, predominantly eat fruits. They complement their fruit diet by occasionally feeding on insects, lizards and crabs.
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For me, a researcher interested in the role of birds in seed dispersal, Narcondam Island offered a unique opportunity. The island does not harbour bulbuls, barbets and fairy bluebirds, which are among the important seed disperser groups on the mainland. It is only home to relatively larger fruit-eating birds like the Narcondam hornbill, Imperial-Pigeons, koel and a small population of hill mynas.
Having spent 14 years studying hornbills in the North-East, studying the unique Narcondam hornbill was always on my mind. The bird community-wide seed dispersal study that I had led in Pakke in Arunachal Pradesh between 2016-18 had raised some important questions—like what role large frugivores (creatures that live mainly on raw fruit, nuts and seeds) would play in the absence of small frugivores. Our Pakke study had demonstrated that the small frugivores were key for a diverse array of small- and medium-seeded plant species. Narcondam Island offered the perfect stage to find answers to some of those questions.
Additionally, the small island with seven-eight frugivore species enabled us to assess the relative role of hornbills in seed dispersal, something that is difficult at a site like Pakke, where hornbills share the fruits with almost 50 other avian frugivore species apart from mammals.
Between December 2019 and February 2020, our team spent time studying this magnificent species. We were interested in determining how many hornbills and other birds were present on the island, and their ecological role in maintaining the diversity of plants.
The Narcondam hornbill ancestor that first colonised the island is likely to have come with a gut full of seeds from Myanmar, inadvertently dispersing seeds of different species of their food plants on the island. Arguably the closest extant relative of the Narcondam hornbill can be found in Papua New Guinea and adjacent islands—it’s a backstory we are hoping to piece together in time.
We walked trails along the treacherously steep slopes of the island from sea level to the mountain top (710m above sea level) in the mornings and afternoons. We recorded the species and number of individuals of different birds and the perpendicular distances of the birds from the trail. This information was analysed using a statistical technique called “distance” analysis to determine bird density and population.
We estimated that the population of the Narcondam hornbill is around 1,000 birds. Yes, there are only 1,000 individuals of this species in the entire world! On the island, however, the hornbill was the commonest fruit-eating bird. It was four times more abundant than the koel, the second-most abundant fruit-eating bird on the island. The other important fruit-eating birds on the island include the Green and the Pied Imperial-Pigeons.
When we systematically collected information on what species of fruits the different birds eat, we got some fascinating insights. There were at least 27 fleshy-fruit plant species that were fruiting on the island while we were there. This included different species of figs and a palm called the Caryota mitis, which resembles the fish-tail palm found in mainland India.
Like those present on Narcondam Island, most plant species in tropical forests are dependent on animals for seed dispersal. Scattering the seed away from the parent plant is important. If all the seeds fell under the parent tree, there would be higher chances of diseases, fungal infections and predation by rodents attracted by the stash of fallen seeds, as well as competition amongst the saplings to germinate.
In the two months we were there, of the 27 different fruiting plant species, we found the hornbills dispersing seeds of at least 22 species—more than any other bird found on the island. And we saw only the hornbill dispersing the four large-seeded plant species, Caryota mitis, Endocomia macrocoma, Pycnarrhena lucida and Planchonella longipetiolata.
Given their hyper abundance, hornbills are the dominant seed dispersers on the island, and by singly dispersing seeds of different large-seeded plant species, they play a critical role in maintaining plant diversity on the island.
We found that Narcondam Island has the highest density of hornbill food plants in the world, likely explaining why there are so many hornbills on the island. Interestingly, densities of fig plants are also the highest in the world, making the island heaven for fruit-eating birds. Figs are the mainstay of the diet of hornbills and other fruit-eating birds on the island.
Allan Octavian Hume discovered the Narcondam hornbill in 1873. However, we continue to find something unique and fascinating about this species nearly 150 years later. Unfortunately, there is still a lot to be discovered about other islands in the Andaman and Nicobar archipelago which harbour other unique species. We know little of their ecology.
Dedicated studies on the ecology of terrestrial biodiversity of tropical islands in India is limited. We do not have information on the populations of endemic species, their vulnerability to human-induced changes like habitat degradation, loss and hunting.
By destroying island habitats around the world, introducing non-native species, and hunting, we have lost many species. Globally, the bulk of bird extinctions have happened on islands—from the large dodos and moas to the small wrens in New Zealand. Given their size and distinctiveness, we have exactly one chance to save these islands—and we should not squander it.
Rohit Naniwadekar is a scientist at the Nature Conservation Foundation, with an interest in research on and conservation of endangered hornbills.
LAST UPDATED05.04.2021 | 03:17 PM IST